1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a printing apparatus capable of multiple color applications in a single plate cylinder rotation.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress printing, gravure printing and offset lithography. All of these printing methods require a plate, usually loaded onto a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing plates, in contrast, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the plate by a doctor blade or similar device prior to contact between the plate and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording medium; the plate first makes contact with a compliant intermediate surface called a blanket member which, in turn, applies the image to the paper or other copying medium. In typical rotary press systems, the recording medium is attached to an impression cylinder, which brings it into contact with the blanket member.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or xe2x80x9cfountainxe2x80x9d) solution to the plate prior to inking. The fountain solution prevents ink from adhering to the nonimage areas, but does not affect the oleophilic character of the image areas.
The plates for an offset printing press are produced photographically or through digital imaging. Traditionally, plates have been affixed to the plate cylinders of the press by means of clamps and the like. More recent systems, however, eliminate the chore of removing and replacing spent plates by locating a continuous supply of imageable plate material within a cavity within the plate cylinder. Each time a printing job is completed, fresh plate material is advanced around the cylinder to replace the spent segment.
Photographic platemaking processes tend to be time-consuming and require facilities and equipment adequate to support the necessary chemistry. To circumvent these shortcomings, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink-jet equipment that directly deposits ink-repellent or inkaccepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to physically alter the topology of
the plate blank, thereby producing xe2x80x9cdotsxe2x80x9d which collectively form a desired image (see, e.g., U.S. Pat. No. 4,911,075, co-owned with the present application and hereby incorporated by reference). For example, the plate material may be imaged utilizing an imager comprising a laser device that either ablates one or more layers of plate material or physically transforms a surface layer. See, e.g., U.S. Pat. No. 5,339,737 co-owned with the present application and hereby incorporated by reference.
In most conventional presses, if a press is to print in more than one color, a separate printing member corresponding to each color is required. The original image is transformed into a series of imagewise patterns, or xe2x80x9cseparations,xe2x80x9d that each reflect the contribution of the corresponding printable color. The positions of the printing members are coordinated so that the color components printed by the different members will be in register on the printed copies. Each printing member ordinarily is mounted on (or integral with) a xe2x80x9cplatexe2x80x9d cylinder, and the set of cylinders associated with a particular color on a press is usually referred to as a printing station. Typically each such station typically includes an impression cylinder, a blanket member, a plate cylinder and the necessary ink (and, in wet systems, dampening) assemblies. The recording medium is transferred among the print stations sequentially, each station applying a different ink color to a material to produce a composite multicolor image.
Central impression designs reduce the number of press components and printing errors arising from paper handoff by minimizing the number of times a sheet is actually transferred. The sheet may, for example, be withdrawn from a bin and affixed to the central impression cylinder in a single operation, and stripped from the cylinder only after traversing all printing stations. In this way, misregistration errors are substantially reduced, since the opportunity for paper slippage between stations is removed. Furthermore, any errors resulting from initial paper handling are not amplified, since the orientation of the paper with respect to the printing stations remains essentially fixed.
Unfortunately, even with central impression designs, each color component requires a separate and unique printing station. Accordingly, the configuration of a conventional multicolor press is comparatively complex, expensive and large.
In accordance with the invention, use of a blanket member having a release surface with a transfer rate approaching 100% facilitates a press design utilizing a single blanket member to transfer multiple colors of ink onto a recording medium. As a result, a multicolor press may include a single, large plate cylinder having multiple image regions, a single blanket member and a single impression cylinder. This approach is substantially simpler than traditional designs, which, as noted above, contemplate a separate printing station (with its own plate cylinder, blanket member and impression cylinder) for each color and complex xe2x80x9chandoffxe2x80x9d mechanisms to transfer recording media sequentially among the printing stations.
Accordingly, in a first aspect, a press in accordance with the invention comprises a plate cylinder having a plurality of image regions, one or more imagers for placing a lithographic image on the plate material at each of the image regions, one or more inking mechanisms for transferring a different color of ink to each of the images, and a single blanket member in rolling contact with the plate cylinder for sequentially receiving the ink from each of the images as the plate cylinder rotates. The blanket member receives successive applications of ink and transfers these to a recording medium, which is typically pinned to an impression cylinder in rolling contact with the blanket member. Again, because of the high release efficiency of the blanket member, the same member is capable of receiving and transfering sequential applications of differently colored ink.
In general, if the plate cylinder has a diameter D and N image regions, the diameter of the blanket member and the impression cylinder will be D/N. It should be noted that this relationship does not require a cylindrical blanket member; for example, the blanket member may be in the form of a belt with an exterior length D/N. In one embodiment, the diameter of the plate cylinder is four times that of that of the blanket member and the press contains four image regions evenly distributed about the circumference of the plate cylinder.
In preferred embodiments, a multicolor press in accordance with the invention contains multiple winding mechanisms within the plate cylinder, which are selectively actuable so as to pay out material across the cylinder segments corresponding to the image regions. For example, the winding mechanisms may be differently geared to cylinder rotation, such that rotation of the cylinder in a first direction advances material from a first winding mechanism across a first circumferential portion of the cylinder to a second winding mechanism; while rotation of the cylinder in the opposite direction advances material from the second winding mechanism across a second circumferential portion of the cylinder (which may, for example, be diametrically opposed to the first cylinder portion) to the first winding mechanism. Alternatively, material advancement may be achieved by means of one or more dedicated motors rather than mechanical coupling to cylinder rotation.
In accordance with these embodiments, therefore, at least two winding mechanisms are desirably distributed around a cylinder. Each winding mechanism includes rotatable supply and take-up spools within the cylinder, and means for winding material onto the take-up spool. The supply spool of each winding mechanism is configured to dispense recording material over a travel path extending around the cylinder to the take-up spool of an adjacent winding mechanism. Accordingly, material may be advanced from a selected winding mechanism (with the remainder inactive).
In a second aspect, the invention comprises a printing method. A plate cylinder having a plurality of image regions is provided, and plate material is disposed on the plate cylinder. A lithographic image is applied to the plate material at each of the image regions. During printing, ink is transferred to each of the images, with each image receiving ink of a different color. A single blanket member in rolling contact with the plate cylinder sequentially receives the ink from each of the images as the plate cylinder rotates and transfers the ink to a recording medium.